Fuel cells using a polymer electrolyte generate electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen, such as air. The fuel cell basically includes a polymer electrolyte membrane that selectively transports hydrogen ions and a pair of electrodes disposed on both faces of the electrolyte membrane. The electrodes comprises a catalyst layer mainly composed of carbon power with a platinum group metal catalyst carried thereon and a gas diffusion layer which has gas permeability and electronic conductivity and is formed on the outer face of the catalyst layer.
In order to prevent leakage of the supplied fuel gas and oxidant gas or mixing of the two gases, gas sealing members or gaskets are arranged on respective outer circumferences of the electrodes across the polymer electrolyte membrane. The sealing members or the gaskets are integrated with the electrodes and the polymer electrolyte membrane beforehand. This is called MEA (electrolyte membrane-electrode assembly). Electrically conductive separator plates are disposed outside the MEA to mechanically fix the MEA and to electrically connect adjoining MEAs with one another in series. Gas flow channels, through which reaction gases are supplied to the electrodes and a generated gas and excess gases are flown out, are formed in specific parts of the separator plates that are in contact with the MEA. The gas flow channel may be provided independently of the separator plate, but the general arrangement forms grooves on the surface of each separator plate to define the gas flow channel.
Through holes are formed in the separator plate to supply the fuel gas or the oxidant gas to the gas flow channel in the separator plate. The inlet and the outlet of the gas flow channel communicate with these through holes. The supply of the reaction gas is distributed via the through hole to the respective grooves of the gas flow channel. The through hole formed to supply the reaction gas to the respective grooves of the gas flow channel is referred to as the manifold aperture.
The fuel cell produces heat during its operation. The fuel cell should thus be cooled down with cooling water, in order to maintain the cell in a favorable temperature condition. A cooling unit for the flow of cooling water is typically provided between adjoining separator plates at every 1 to 3 unit cells. In many cases, the cooling unit is a cooling water flow channel formed in the rear face of the separator plate. A stack of fuel cells of typical construction is obtained by successively laying 10 through 200 unit cells one upon another to a cell laminate, which includes the MEAs, the separator plates, and the cooling units, disposing end plates across the cell laminate via collector plates and insulator plates, and clamping the both end plates with clamping bolts.
For easiness of processing, in the polymer electrolyte fuel cell, the separator plate generally has a square or rectangle contour. An identical shape is applied to manifold apertures for a fuel gas containing hydrogen, manifold apertures for an oxidant gas containing oxygen, and manifold apertures for cooling water. This enables sealing members and other peripheral members to be shared. These manifold apertures are arranged to be practically symmetrical about the centerline and the diagonal of the separator plate.
When the contour of the separator plate is square or rectangle as in the case of the prior art polymer electrolyte fuel cells, the orientation of the separator plate can not be specified according to the contour in the course of assembling the cell stack. It is accordingly difficult to check the surface and the rear face of the separator plate. In the case where an identical shape is applied for the respective manifold apertures and the manifold apertures are arranged in a line symmetrical layout, the separator plate does not have any specific orientation of the surface and the rear face. For the CO poisoning resistance, different noble metal catalysts are generally used for the anode and the cathode in the MEA. The proper orientation of the surface and the rear face of the MEA is thus of significant importance. The MEA basically has the same contour and the same pattern of the manifold apertures as those of the separator plate. The MEA accordingly does not have any specific orientation of the surface and the rear face. There is accordingly a high possibility that the anode and the cathode are mistakenly set in the course of assembling the cell stack.
Each manifold aperture plays an important role of distributing the reaction gas or cooling water to the corresponding flow channels in the respective separator plates of the cell laminate. The ratio of the opening area of each manifold aperture to the total sectional area of each corresponding flow channel in the separator plates affects the flow rate of the fluid. The excessively small flow rate has a significant effect on the dynamic pressure and remarkably worsens the distribution to the respective separator plates. The prior art technique applies an identical shape to the respective manifold apertures, in order to maintain the symmetry of the separator plate. This technique, however, makes it difficult to attain the adequate ratio of the opening area of each manifold aperture to the total sectional area of each flow channel.